The human airways are daily confronted with at least 7-8 cubic meters of air and there is an advanced biological system to detoxify inhaled particles and gases. The first line defense against inhaled material is the Respiratory Tract Lining Fluid (RTLF), covering all the airways, among other thing containing several important antioxidant systems. Another important component of the RTLF is the surfactant, containing compounds for decreasing surface tension but also taking part in the innate immunity.
The composition of RTLF has been shown to change in inflammatory conditions of the airways. When the balance between anti-oxidants in RTLF and inhaled oxidants is disturbed, oxidative stress will initiate an inflammatory process. This inflammatory process, although very variable, is a major early event which is common in the development of most respiratory diseases, from asthma to lung cancer.
The patho-physiological processes leading to all respiratory diseases are so far not fully understood. One reason behind this is that those processes are difficult to monitor in humans. To evaluate the effect of for example various exposures, the available methods have been limited to measurement of lung-function, exhaled nitric oxide, induced sputum or analysis of broncho-alveolar lavage (BAL) or biopsies from bronchoscopy.
Those existing methods are either too invasive i.e. bronchoscopy, and thereby not applicable in larger studies which is warranted as susceptibility to different exposures are highly variable. Besides, both bronchoscopy and induced sputum are associated with certain risks, especially in sensitive populations as in those with pre-existing cardiopulmonary disease or asthma. Nitric oxide in exhaled air seems to a large extent solely to reflect an allergic inflammation and is therefore of limited value when studying other forms of airway disease. Lung function, on the other hand, is rather harmless to the studied patient but gives no information on underlying mechanisms of disease.
Other methods used include in-vitro studies, which only allow limited generalizations to the complex environment of human airways. The same is to a large extent true for animal studies, where—although genetic concordance to humans is high—the expression of various genes differs substantially.
Lately a new method has been introduced, namely, collection of exhaled breath condensate (EBC) i.e. exhaled water vapour that is condensed by the means of low temperature, where both volatile and non-volatile compounds have been identified. The non-volatiles found in EBC are believed to originate from particles formed within the airways. These particles are generated in the respiratory system while breathing, speaking or coughing and have been observed and, until now, studied mainly because such particles may serve as vehicles for transport of infectious material. How these particles are formed is still unknown, but a plausible mechanism may be through turbulent flow of the exhaled air in the central airways where the cross section area of the bronchi decreases substantially. A second hypothesis is that particles are formed from the RTLF when airways open up in the peripheral lung. In disease, the formation of particles may be enhanced due to increased turbulent flow and/or changed physical properties of the RTLF. An example of this is given in WO 02/082977.
The collection of exhaled breath condensate (EBC) is connected with a number of serious methodological difficulties such as dilution with water resulting in very low concentrations of the substances of interest, high contamination with substances originating from the oral cavity, high intra-individual coefficient of variation and a very inefficient way to sample the non-volatiles found in EBC.
Hence there is a need for better non-invasive methods to detect and monitor adverse health effects of the respiratory system. One, until now unexamined, way to overcome some of the methodological difficulties connected with analysis of EBC would be to directly sample and analyze the exhaled particles. The ability to determine amount and size of the collected particles will also give specific information about the status of the respiratory tract.
Measurement of Distribution of Particle Fractions of Different Sizes
There are only a few studies published examining exhaled droplets (i.e. particles).
Papineni and Rosenthal [J Aerosol Med 10(2):105-16] and Edwards et al. [Proc Natl Acad Sci USA 101(50):17383-8] measured a number of concentrations of exhaled particles in humans and described that it varied considerably between subjects but the concentrations were generally much lower than found in typical indoor air. Some information regarding size distributions of exhaled particles were also presented. It must be assumed that the main constituent of the droplets is water and thus, particle size should vary quickly with varying relative humidity (RH) of the surrounding air. The procedures to investigate the influence of RH used by Papineni and Rosenthal are not convincing since an IR-lamp was used to heat the air to change RH. Edwards et al. did not consider RH in a serious way in their investigation. Particle size was either invoked by indirect methods, e.g. microscopy of dried droplets or by light scattering methods with low size resolution. Thus, this state of affairs warrants further investigation of the variability in concentration and size distribution of exhaled aerosols.
There has also lately also been increasing interest in human aerosol formation mainly in the scope of the potential to detect their infectious potential. US 2005/0073683 and Anal. Chem. 2005, 77, 4734-4741 describe a real-time detection method and system for identifying preformed aerosol particles. The method described is aiming at detecting aerosols containing contagious material or “threat agents” on-line, by comparing their positive and negative mass-spectra with reference spectra which also will be developed. That method is not developed to diagnose or monitor human airway conditions and is markedly less sensitive which hinder detection of substances in very low concentrations, such as in the exhaled particles.
There is a lack of methods for easy monitoring of the airways. Invasive procedures, such as bronchoalveolar lavage and sputum induction, can be harmful to the patient and do not allow frequent sampling.